This application is related to a co-pending application entitled ZEROING CIRCUIT Ser. No. 492,424 filed Aug. 2, 1974.
The invention relates to detecting a parameter of a component of a gas in a chromotograph, and more particularly to detecting a signal level that is indicative of the parameter.
Electrical signals are often used to carry parameters relating to many processes. Several aspects of a signal are used to represent the values of the parameters; an important one of these is the highest level which a signal attains during a particular interval of time. In gas chromotography for example, in which a gas containing molecules of different types is separated into its component parts, a signal is generated whose level is proportional to the number of molecules that pass a sensor.
The chromotograph uses a resistive medium, such as small pieces of firebrick coated with a heavy oil, to separate the gas components. A standard-sized sample of gas is carried through the medium at a constant rate by a gas such as helium. The resistive medium resists the passage of the sample gas molecules in proportion to their size. As a result the molecules of a particular size tend to bunch together and exit from the resistive medium about the same time. The smaller molecules exit first, followed by groups of molecules of progressively larger sizes. Each group of a particular size requires a known amount of time, called the elution time, to pass through the resistive medium. Each group is distributed over a small interval of time, usually in a normal or Gaussian distribution. Since the shape of the distribution is known and if the Gaussian peaks are symmetrical, the number of molecules in a group can be determined from the peak value of the distribution.
The passage of the molecules from the resistive medium is commonly sensed by a thermistor. The passage of the gas molecules over the thermistor alters the temperature of the thermistor, which in turn alters its resistance. The varying resistance is used to modulate an electrical signal. The shape of the signal will thus be related to the distribution of the molecules in a particular group separated by the resistive medium. The peak value of the signal can then be determined by means of the invention and will be a measure of the amount of the component represented by the group in the gas sample.
A peak detector commonly used in chromotography is electro-mechanical. In this device the signal from the detector is fed into the recorder and a potentiometer which varies a voltage level is connected by a shaft to the recorder mechanism. A friction switch is activated when the signal level from the detector begins to rise and remains open due to the friction as long as the signal continues to rise. However, when the signal reaches its peak and begins to fall, the frictional force subsides and the switch closes. The closing of the friction switch causes the voltage level at the output of the potentiometer to be inserted into a storage device. It is apparent that such a detector is inaccurate and difficult to maintain in proper calibration. In addition, its mechanical parts are subject to wear and tear.
It is therefore an object of the invention to provide a new and improved detector for a chromotograph that overcomes the above and other disadvantages of the prior art. Other objects become apparent as the invention is described in detail.